The information was obtained through a Freedom of Information Act request submitted three years ago by equine cruelty investigator Julie Caramante.
Animals' Angels assisted Caramante in obtaining the documents, and they are now working with Animal Law Coalition to assess and disseminate the information.
The documents include almost 900 photographs.
"I've been an equine cruelty investigator for a number of years," said Caramante, "and I've witnessed many incidents of animal cruelty but nothing could prepare me for these images."
Horse slaughter in the US ended in 2007 after the three remaining plants in Texas and Illinois were closed by state lawmakers and the courts. There is a federal bill pending in Congress that would prevent horse slaughter from resuming in states without laws prohibiting it.
However, the end to the slaughter industry in the US has seen horses shipped in increasing numbers to plants in Mexico and Canada.
Sonja Meadows, executive director of Animals' Angels, said, "We now know from these new documents that being on US soil does not make horse slaughter humane or better."
Horse slaughter is not euthanasia, she said, and "definitely not a humane end".
Meadows is hopeful that the documentation will compel the new Congress to act swiftly to pass a federal ban on the transport and slaughter of American horses.
"In 2006 the House voted by a wide margin, 263-146, to ban horse slaughter. But the session ended before the Senate could vote," she said. "Now, this new evidence removes any doubt. We must act quickly. We cannot allow horse slaughter to continue any longer."